Over the last few decades, the electronics industry has undergone a revolution by the use of semiconductor technology to fabricate small, highly integrated electronic devices. The most common semiconductor technology presently used is silicon-based. A large variety of semiconductor devices have been manufactured having various applicability and numerous disciplines. One such siliconbased semiconductor device is a metal-oxide-semiconductor (MOS) transistor.
The principal elements of a typical MOS semiconductor device are illustrated in FIG. 1. The device 100 generally includes a gate electrode 103 of doped polycrystalline silicon, which acts as a conductor, to which an input signal is typically applied via a gate terminal 104. Heavily doped source and drain regions 105 and 106 are formed in a semiconductor substrate 101 and are respectively connected to source and drain terminals 107 and 108. A channel region 109 is formed in the semiconductor substrate 101, which is often referred to as the body region of the transistor, beneath the gate electrode 103 that separates the source and drain regions 105 and 106 when the appropriate voltage is applied to the gate. The body region is typically lightly doped with a dopant type opposite to that of the source/drain regions 105. The gate electrode 103 is physically separated from the semiconductor substrate 101 by a gate insulating layer 110, typically an oxide layer such as SiO.sub.2. The insulating layer 110 is provided to prevent current from flowing between the gate electrode 103 and the source and drain regions 105 and 106, the body region 101, or the channel region 109. A body contact region 112 that is heavily doped with a dopant opposite that of the source and drain regions 105 and 106 is also formed to connect the body region to a voltage. The body contact region 112 is connected to a grounded body terminal 114, for example.
In operation, an output voltage is typically developed between the source and drain terminals 107 and 108. When the correct input voltage is applied to the gate electrode 103, the electric field below the gate 103 induces the channel region 109. By varying the voltage on the gate 103, and hence the electric field, it is possible to modulate the conductance of the channel region 109 between the source and drain regions 105 and 106. In this manner an electric field controls the current flow through the channel region 109. This type of device is commonly referred to as a MOS field-effect-transistor (MOSFET).
The ability of both n-channel and p-channel MOSFETs to withstand over-voltage conditions is important for device reliability. MOSFETs connected to input or output pads are most likely to experience an over-voltage condition. If the drain-to-source breakdown voltage (BV.sub.dss) of a MOS transistor is exceeded, the resulting current flow may damage or destroy the device. In FIG. 1, the drain is reverse biased and the resulting depletion region 120 extends under the gate 103.
Damage can be caused by breakdown occurring at or near the surface of the transistor. The device can be damaged in two ways. First, energetic electrons and holes that are generated when breakdown occurs can be injected into the dielectrics in the vicinity of the drain-to-body junction, particularly the gate dielectric 110, thereby changing the apparent threshold voltage of the MOSFET. The change in apparent threshold voltage can result in a higher current flow for the same applied gate voltage, which can destroy a transistor if the reduction in the apparent threshold voltage is sufficiently large. Second, heat that is generated near the surface as a result of the localized power dissipation can alter the characteristics of the materials in the area, for example, silicon, metals, and dielectrics. Increased conductivity can result and change the performance of the MOSFET.
Therefore, a semiconductor structure that addresses the above identified problems associated with drain-to-body breakdown is desirable.